1. Field of the Invention
The present invention relates to a multibeam radar system for obtaining information such as the presence or absence of an object (target), the distance to the object, and the speed relative to the object by scanning beams of ultrasonic waves or radio waves in numerous directions, sending the beams, and receiving echo waves from the object.
2. Description of the Related Art
Two known means are available to scan beams of ultrasonic waves or radio waves within a certain angular range within a certain plane. One means is a mechanically scanning radar system using a rotated antenna. The other means is an electronically scanned radar system for making electronic scans. Waves are transmitted from such a radar system and echo waves are reflected from the object. Various kinds of information, (i.e., the intensity of received signal, the presence or absence of the object indicated by the intensity of received signal, the distance to the object, and the speed relative to the object) are contained in the echo waves. Since the aforementioned scan is often performed within a given angular range within a horizontal plane, the direction of scan is also known as the azimuth direction.
In the case of a radar system installed on a vehicle, fixed structures such as guardrails exist on roads and are close to a vehicle to be detected. Therefore, it is necessary to discern the features of the shape of the object producing echo waves and to erase unwanted images. Consequently, rough detection of the shape or contour of the object is needed, in addition to the detection of the presence or absence of the object. When the shape or contour is discerned, the azimuthal resolution is of importance.
In any of the aforementioned radar systems, however, if millimetric waves are used, the transmitted beam has a considerable width and so the azimuthal resolution can deteriorate. The azimuthal resolution can be effectively enhanced by reducing the beam width. Although it is possible to narrow the beam by making the reflector antenna larger, restrictions are imposed on the increase of the size of the antenna because it must be installed on a small-sized moving object such as a vehicle. The beam width can be reduced by using higher transmission frequencies but the transmission loss increases as higher frequency is employed. In this way, limitations are placed on enhancement of the azimuthal resolution where the method making use of narrowing of the beam is exploited.
On the other hand, in the case of a laser radar, it is possible to narrow the beam. However, it is vulnerable to natural environmental variations such as fog and rain. Therefore, the laser radar cannot meet the requirements imposed on the vehicular radar system.
A deformed deconvolution method as described in Japanese Patent Laid-Open No. 125484/1992 is known as a method of enhancing the azimuthal resolution at a considerable beam width. In this deformed deconvolution method, a beam having a finite width is treated as an apparatus function having a spatial spread. An object existing in a scanned space is treated as a signal source having a spatial spread and attributes, i.e., positions in the azimuthal direction and intensities of reflected signals. In this way, the deconvolution method which is known as a method of processing signals in a time-and-space domain is extended to a method of processing signals inside a real space. That is, inverse calculational processing of the apparatus function is applied to received, reflected waves emanating from plural signal sources which are sporadically located in the real space. In consequence, the azimuthal resolution is improved.
The optimum scanning method for enhancing the azimuthal resolution is next discussed. A mechanically scanning radar system needs a mechanism for rotating an antenna and a device for driving the rotating mechanism and so the system is large in size. This makes it difficult to install the system on a relatively small-sized moving object such as a vehicle. In addition, since the object on which the system is installed moves, the rotating mechanism and its driving device must be sturdy enough to withstand vibrations that the moving object undergoes. Moreover, the moving object carrying the radar system moves at a relatively high speed and, therefore, the radar system is required to attain a scanning speed corresponding to the moving speed. The mechanically scanning radar system has the advantage that it can secure relatively high azimuthal resolution, but it is difficult for the rotating mechanism to accommodate itself to the scanning speed required by the moving object. For these reasons, the electronically scanning radar system is more practical than the mechanically scanning radar system.
A typical example of the aforementioned electronically scanning radar system is the time-sharing electronically scanning system previously proposed by the present Applicant in Japanese Patent Laid-Open No. 264728/1995. In this system, the same number of antennas as the number of beams are arrayed. Since a large number of antennas are used, the whole system is bulky. Hence, this system is not as suitable as would be desired as a long-range radar system over a highway. Another antenna already proposed by the present Applicant is disclosed in Japanese Patent Laid-Open No. 273339/1995. This antenna makes it possible to detect a long distance such as about 100 m ahead of a vehicle. The whole of this antenna can be made small. A defocus multibeam antenna is used as this antenna and has the same number of small primary feed horns arrayed as the number of beams. The feed horns reflect incident waves toward a common reflector antenna. A further typical electronically scanning radar system is a phased array radar system comprising an array of small antennas. The phases of signals received by the individual antennas are matched in the azimuthal direction.
However, whether the electronically scanning radar system using the defocus multibeam antenna or the phased array system is employed, transmitted beams more spaced from the center of the scanning range (i.e., spaced closer to both ends of the range) are deformed to a greater extent. This deteriorates the azimuthal resolution and, at the same time, narrows the range in the direction of detection. Since the beams in every direction do not have the same shape, the azimuthal accuracy is not improved even if the deconvolution method described in the above-cited Japanese Patent Laid-Open No. 125484/1992 is used as it is.
Another radar system makes use of an FM radar system. An antenna is located at the origin of a two-dimensional space which is expressed in terms of a polar coordinate system. Objects are sporadically dispersed in this space and reflect signals. These reflected signals are processed appropriately to accomplish accurate measurements. However, there is room for discussion of the order in which the reflected signals are processed.